This invention relates to a method of conditioning a supply of bleed air from an aircraft engine to an apparatus or system using such bleed air, to provide such air in a required condition.
In referring herein to an aircraft engine it is to be understood that engines of the turbine type generally used for military or commercial civil aircraft may also be used in propulsion systems for other means of transport, and the invention is applicable to such other uses of the engines, as well as to their use in aircraft. Further, in relation to aircraft, we include all types thereof whether heavier or lighter than air and manned or unmanned.
The term bleed air as generally understood and as used herein refers to pressurised air which is extracted from an aircraft engine after a or the compressor stage thereof.
It is well known for most types of military and civilian aircraft to utilise bleed air from the aircraft""s engine(s) for air conditioning and other purposes. Having been compressed in the engine(s) the bleed air is normally much too hot to be used directly and therefore has to be thermally conditioned by passing it through at least one heat exchanger. Conventionally the cooling medium in such a heat exchanger is ram air, i.e. ambient air caused to flow through the heat exchanger as a result of the forward speed of the aircraft.
Such use of ram air to cool the bleed air has certain disadvantages. One is that the structure of the aircraft has to be designed to provide a ram air inlet or inlets, which sometimes have to be provided with adjustable doors or shutters to vary the size of the opening they afford: this entails the provision of actuators and controls. Also the internal structure of the aircraft has to provide space for the ram air to pass to and from the heat exchanger. Improvements in heat exchanger design and efficiency has enabled the size of heat exchangers to be reduced, but the use of ram air nevertheless creates aerodynamic drag on the aircraft, adversely affecting speed and fuel consumption. A further disadvantage pertaining to military aircraft is that compression of air at or in front of a ram air inlet opening creates a xe2x80x9chot spotxe2x80x9d which can be detected by infrared-seeking anti-aircraft missiles.
An alternative to the use of ram air, particularly in civil aircraft, is the use of low-pressure air impelled by a fan stage of the engine as the cooling medium, to cool the bleed air in a heat exchanger. Heated fan air downstream of the heat exchanger is discharged overboard.
Whether ram air or fan air is used in a heat exchanger to reduce the temperature of compressor bleed air, a lot of power is wasted by the discharge overboard of heated air. Under certain conditions, possibly 300 kW might be wasted in the discharge of heated air. This also is a disadvantage in the case of military aircraft, since a hot outflow of air can be detected by a heat seeking missile. There are devices whereby the effect of discharge of a large quantity of hot air can be suppressed, but it would be desirable if the discharge could be avoided entirely.
Some further disadvantages associated with the use of bleed air from an aircraft""s engine(s) arise from the fact that the pressure of the bleed air varies as the engine compressor speed varies and/or as the aircraft""s altitude changes. Such variations in pressure could cause operational problems if air conditioning apparatus is subjected to them. Hence there is a requirement for pressure reducing/regulating valves for supplying a generally constant (within operating limits) pressure downstream. Such pressure regulating valves are complex and contain sensitive components. The problem is greater if there are multiple engines and the outputs of bleed air therefrom have to be matched.
It has been proposed in each of U.S. Pat. No. 3,764,814 and U.S. Pat. No. 5,899,085 to pass bleed air to an external turbine which in turn, drives a power generator, to generate electrical power for use in the aircraft in an effort to increase the efficiency of the system, but these proposals do not suggest how to accommodate variations in the pressure of the bleed air arising due to changing operational conditions.
According to one aspect of the present invention, we provide a method of conditioning a supply of bleed air from an aircraft engine to a system using such bleed air, including passing the bleed air to a turbine over which the air is expanded to produce a power output, and regulating the power output thereby to control the condition of the air downstream of the turbine.
Thus by regulating the power output, irregularities in the bleed air pressure are prevented or the effect of them in the system in which the bleed air is to be used, is reduced.
Preferably an electrical generator is driven by the turbine. Then the electrical power output of the generator can be regulated to control the expansion of the air in the turbine.
Aircraft have complex electrical systems in which electrical power thus generated can be utilised. However, it would be within the broadest aspect of the invention if the turbine were to drive an hydraulic pump for example, whose power output could be utilised in an aircraft""s hydraulic system.
The method of the invention may include sensing at least one parameter of the air downstream of the turbine and changing a load on the power output depending upon the sensed parameter thereby to regulate the condition of the air.
The system in which the bleed air may be used may be an air cycle air conditioning system and the bleed air is expanded and cooled as the bleed air passes over the turbine and may thus be used to cool a heat load.
According to a second aspect of the invention, we provide apparatus for conditioning a supply of bleed air from an aircraft engine to a system utilising such bleed air, the apparatus including a turbine providing a power output as a result of expansion of the air, and a controller for regulating the power output and thereby controlling the condition of the air downstream of the turbine.
Preferably the turbine drives an electrical generator, an electrical load on the generator being regulated by a controller in response to at least one parameter of the air downstream of the turbine.
Preferably the sensor for providing an input to the controller is responsive to one or more parameters including, but not limited to, turbine speed, turbine inlet temperature, turbine inlet pressure, turbine outlet pressure, turbine outlet temperature, the controller responding to the or one or more of the inputs by regulating the electrical load to increase or decrease turbine speed so that the pressure of air downstream thereof is maintained in a generally steady state.
The controller may also be responsive to signals relating to operational conditions, such as flight parameters, of which change would result in a change in the operating state of the engine or engines. Thus the controller may integrated with an engine control system and/or an aircraft control system.
There may be a bypass through which at least a proportion of the bleed air can pass to the system where it is to be used other than by way of the turbine or other expansion device. The proportion of bleed air passing through the bypass may be determined by a valve whose operation preferably also is controlled by the controller.